This invention relates generally to filters and molecular sieves and more particularly to methods for producing filers and molecular sieves from semiconductor materials.
Filters and molecular sieves are used to separate constituents of a fluid substance based, respectively, upon the constituent's particulate or molecular size. Filters and molecular sieves have been made from a broad variety of materials including metals, plastics, ceramics, and organic and inorganic fibers and can be made by a variety of methods.
Molecular sieves can be produced by a method known as nuclear track etching. While nuclear track etching can be used to produce molecular sieves in many types of inorganic materials it is most often used to make molecular sieves out of an insulating material such as mica. In this method, a substrate is first bombarded with nuclear particles to produce disturbances or "tracks" within the normal lattice structure of the material and is then wet-etched to produce pores which follow the tracks caused by the nuclear particles.
U.S. Pat. No. 3,303,085 of Price et al, teaches the formation of molecular sieves by a nuclear track etching process. More specifically, Price et al, disclose that the exposure of a mica substrate to heavy, energetic, charged particles will result in the formation of a plurality of substantially straight tracks in its lattice structure and that these tracks can be converted into pores by wet etching the substrate. The resultant structure may be used as a molecular sieve. Price et al. do not, however, teach the formation of filters or molecular sieves from semiconductor materials.
A number of references teach the electrolytic etching of silicon in a hydrofluoric acid solution. For example, German Patent No. 3,324,232 of Foll et al. teaches the etching of a silicon body in a hydrofluoric acid electrolyte where the silicon body comprises the anode and an acid-resistant material such as graphite forms the cathode of an electrolytic cell. The etching process forms a number of honeycomb cells into the suface of the silicon body, thereby increasing the effective surface area of the body. The increased surface area is useful when the silicon body is an element of a solar cell since the efficiency of the cell is directly related to the exposed surface area of the cell.
Electrolytic etching of silicon is also disclosed in Japanese Patent No. 58-140,131 wherein a silicon dioxide layer is formed on the backside of a silicon wafer before the etching process is commmenced. The silicon dioxide layer is said to promote uniform thickness and density in the resultant porouus silicon layer. Other electrolytic etching process are described in U.S. Pat. No. 4,303,482 of Buhne et al. and 4,874,484 of Foell et al.
While many references teach the electrolytic etching of silicon surfaces, none teach the manufacture of filters or molecular sieves by the electrolytic etching of silicon or any other semiconductor material. In fact, all known prior references utilize electrolytic etching of silicon to prepare the silicon for use in electrically active circuitry. It is therefore understandable that the conventional wisdom in this field was that this process is only useful to create pores a few tens of microns deep into the surface of a silicon body. Furthermore, electrolytic etching apparatus of the related art are, in generally, unsuitable for use in the production of porous semiconductor filters and molecular sieves where the pores extend fully through a semiconductor body.
The method of this invention includes placing a semiconductor body within an electrolytic solution comprising HF and H.sub.2 O, coupling a voltage or current source between the semiconductor body and an electrode submersed in the solution, and propagating pores fully through the semiconductor body to create a porous semiconductor membrane. Alternatively, the pores can be partially propagated through the body from a first surface and material can be removed from an opposing second surface to expose the pores.
The pore diameter is dependent upon a number of factors including the dopant level of the semiconductor body, the concentration of the electrolyte and the current density within the electrolytic cell. Asymmetrical filters and molecular sieves can be produced by varying one or more of these operating parameters but are preferably produced by varying either the applied voltage or current.
An important advantage of the porous semiconductor membranes of the present invention is that they are self-supporting structures over a considerable range of pressure differentials and generally do not require any internal or external reinforcement to provide rigidity or strength. Furthermore, the porous semiconductor membranes of this invention can be prepared in virtually any thickness ranging from a few hundred microns or less to several millimeters or more.
In addition to making filters and molecular sieves, the method of the present invention allows the integration of selective fluid sensitive devices on semiconductor wafers. For example, a fluid sensitive device such as an electrochemical sensor can be build into an integrated circuit in conjunction with a porous semiconductor filter. Various micromechanical devices can also be combine on a semiconductor wafer with a porous semiconductor filter.
The semiconductor filters and molecular sieves of this invention have a number of useful characteristics such as a high thermal conductivity, chemical inertness to a great number of substances, good mechanical strength and hydrophilic surfaces.
These and other advantages of the present invention will become clear to those skilled in the art upon a study of the detailed description of the invention and of the several figures of the drawings.